1. Field of the Invention
This invention relates generally to a vehicle powertrain, and, in particular, to control of a power-off downshift of a powershift transmission.
2. Description of the Prior Art
A powershift transmission is a geared mechanism employing two input clutches used to produce multiple gear ratios in forward drive and reverse drive. It transmits power continuously using synchronized clutch-to-clutch shifts. A dual clutch transmission (DCT) has no torque converter
The transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
During a vehicle launch condition in a conventional vehicle whose powertrain includes a powershift transmission, the engine and transmission are concurrently controlled in a coordinated manner to provide acceptable vehicle launch performance. In a powershift transmission vehicle application, providing consistent and acceptable vehicle launch performance can be a rather difficult control problem due to the lack of a torque converter. During a vehicle launch condition in this type of vehicle application, the torque capacity of the transmission clutch and slip across the clutch are carefully controlled in coordination with the engine torque to provide the desired vehicle response.
During a power-off condition the coastdown 2-1 sync speed of first gear could be above engine speed. Depending on the engine braking rate, the target gear layshaft speed may decelerate quickly. If the layshaft speed were lower than engine speed, synchronizing those speeds could cause gear lash crossings along with coastdown bobble.
A need exists in the industry for a control that avoids gear lash during a power-off downshift in a powershift transmission.